Page:Handbook of Ophthalmology (3rd edition).djvu/30

24 RANGE OF ACCOMMODATION. results of examinations in diiferent individuals, or in the same individual under diiferent circumstances. Donders provided this standard in a very simple way by comparing the result of accom- modation to the optical value of a convex lens, which, if placed before the relaxed eye, would have accomplished the same as the accommodation does.

Let the lens X, in Fig. 7, be such that parallel rays striking its surface are focused at /. A screen at / would show distinct pic- tures of distant objects, just as a landscape camera obscura does.

Fig. 7.

If now the instrument be adjusted for some nearer point upon the axis, say at p, then, according to the rule, the distance between L and / would be increased ; if, however, this distance cannot be changed, there is still a means by which a distinct image of jj can be cast at f • for this purpose it is only necessary to combine with the lens L an auxiliary meniscus, L', which has such a focal length that rays proceeding from p, after their refraction in L', become parallel ; they then fall parallel upon X, and, according to the original supposition, are focused at /; now if, for instance, the distance from the point p to L' is 4 inches, then the convex me- niscus must evidently have a focal length of 4 inches ; its optical value would be expressed by +. The same principles can be applied to the eye. If before an emmetropic eye with relaxed accommodation we place a convex meniscus of +, the eye is thus adjusted for a distance equal to the focal length of tliis lens, that is, 4 inches. If the near point of an emmetropic eye is 4 inches distant, then the accommodation does for this eye just what was done by the convex meniscus for the relaxed eye. If we represent the accommodation by the letter A, we have as the measure of the accommodation of an eye whose far point is at an infinite distance, and whose near point is at 4 inches, the expres- sion x=5. The optical value of the accommodation is thus best expressed by the principal focal distance of that convex lens which, added to the condition of refraction in the eye, would com-